Vital signs of a person, for example the heart rate (HR) or respiratory rate (RR), can serve as a powerful predictor of serious medical events. For this reason, the respiratory rate is often monitored online in intensive care units or in daily spot checks in the general ward of a hospital. A non-invasive respiratory rate measurement can be accomplished optically with the help of a stationary video camera.
A video camera captures the breathing movements of a patient's chest in a stream of images. The breathing movements lead to a temporal modulation of certain image features, wherein the frequency of the modulation corresponds to the respiratory rate. Examples of such image features are the average amplitude in a spatial region of interest (ROI) around the patient's chest, or the location of the maximum of the spatial cross-correlation of the ROI in subsequent images. The quality and reliability of the obtained vital sign information is largely influenced by the quality of the input video data, in particular the image contrast and the appropriate selection of the ROI. In particular, a manual selection of the ROI is time-consuming. Further, movements of the patient that are not related to vital signs disturb the measurement.
EP 2 380 493 A1 discloses a respiratory motion detection apparatus for detecting the respiratory motion of a person. An illuminator illuminates the person with an illumination pattern and a detector detects the illumination pattern on the person over time. The illumination pattern deforms significantly with slight movements of the person. This deformation is analyzed over time in order to determine the respiratory movement of the person. The area of pattern projection has to be adjusted on the patient, preferably on the patient's chest. The disclosed apparatus increases the reliability of the respiratory rate measurement at the additional cost of an active illumination unit for pattern projection.